Differences Between OBS and CDI in Aircraft

Since the dawn of aircraft travel, navigation-based systems have been one of the most important features within aircraft, allowing pilots to traverse the sky with ease. Equipped with a number of instruments that enable pilots to navigate large and small aircraft alike, this blog will cover two of the most important types. The first instrument is the Course Deviation Indicator (CDI), and the second is the Omnibearing Selector (OBS).

Both located within the cockpit, a CDI and OBS provide aircrew with critical data during flight. CDIs are standalone instruments that show an aircraft’s deviation from a VOR radial that is selected using the OBS knob positioned on the instrument itself. While their association may be confusing, it is important to note that they perform distinct functions.

To begin, CDIs are either used for VOR navigation or serve as a component within a Horizontal Situation Indicator (HSI). A CDI used for the former reason will either have crosshairs that present vertical guidance for an Instrument Landing System (ILS) or a vertical needle referred to as a CDI needle. Furthermore, CDIs will typically have an OBS knob affixed in the bottom left of the instrument, changing the outer ring of the CDI to determine what VOR radial to analyze your position against.

It is important to note that the CDI alone will not display any data about the current heading, only the VOR radial that is tuned to the OBS will. Essentially, it indicates what the deviation from the tuned radial is at that particular moment. Keep in mind that pilots must have a good level of understanding to interpret the information provided by the CDI as there is no heading information displayed within the instrument.

CDIs, on their own, have one major disadvantage: reverse sensing. As CDIs only provide pilots the deviation with regard to a predetermined radial position that has been selected with the OBS, at any given point, two different radials of the same VOR can be tuned. Usually, the center needle will display a “to” indication and a “from” indication.

If you are tracking with a “from” indication, you will have correct sensing, meaning that the needle is veered to the right. As such, you are to the left of the course and must correct your path to the right. In the case that the CDI shows a “to” indication, you have reverse sensing, meaning that you are to the right of the course and must correct the aircraft to the left. 

OBS, on the other hand, tells the pilot information about the station. For instance, it can let the pilot know what course to fly, whether that is directly toward or away from the VOR. That being said, it becomes clear that both instruments serve important roles. As such, if you find yourself in need of aircraft flight control surfaces or navigation-based systems such as navigation radio parts, VOR navigation components, or other related instruments, rely on Sourcing Streamlined.

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November 2, 2022

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